FIG. 1 shows a typical flatbed scanner 100. In operation, an original item 101, such as a photograph or document, is placed on platen 102. The scanner constructs a digital image of the original item. A digital image is an ordered array of numerical values representing the brightness, color or both of locations on the original item. When the digital image is properly interpreted and displayed, a representation of the original item is recovered. FIG. 2 shows one conventional internal architecture of scanner 100. A carriage 201 containing optics and sensor electronics is swept beneath platen 102 by motor 202. Light generated by lamp assembly 203 reflects off of original item 101 and into carriage 201, where the distribution and intensity of the reflected light is sensed. Typically, lamp assembly 203 illuminates a narrow strip of original item 101, and the light is sensed one line or a few lines at a time as carriage 201 moves under platen 102. Digital values representing the light distribution and intensity are generated by electronics inside carriage 201 working in concert with electronics on controller 204. Carriage 201 and controller 204 are connected by cable 205. The resulting digital data is typically transmitted to a computer for storage, editing, or other uses.
FIG. 3 shows one type of conventional optical path that may be used inside of carriage 201. (The structure of carriage 201 has been removed for clarity.) Light 311 from lamp 301 reflects from original item 101. Light from a narrow “scan line” 302 finds its way into carriage 201, reflects from a series of mirrors 303, 304, and 305, and is gathered by lens 306. Light path 310 indicates the volume swept by the image-forming light. The light is redirected by lens 306 to form an image 307 of scan line 302 on a sensor 308.
Sensor 308 typically comprises one or more rows of photosensitive sites, sometimes called photosites or pixels, and a set of charge coupled devices (CCDs) for storing electric charges generated when light impinges on the photosites. As such, sensor 308 is typically called a CCD sensor, or simply a CCD. CCD 308 is mounted on a printed circuit board 309. The system of FIG. 3 is sometimes called a reduction optics system, because the image 307 is typically reduced in size as compared with scan line 302. This kind of optical system has the advantage that it provides good depth of field. That is, objects a significant distance above platen 102 can still be imaged with good clarity. However, a reduction optics system is often bulky, and it is difficult to make a compact scanner using a reduction optics system as shown in FIG. 3.
Scan line 302 is not a line in the mathematical sense, but has some width. The actual width depends on the magnification of the optical system, the size of the pixels on sensor 308, how many rows of sensors are present on sensor 308, and if there are multiple rows, the spacing between the rows. It is convenient to refer to a “scan line” since scan line 302 is very narrow in relation to its length, even though several parallel rows of pixels may sense light from scan line 302.
In many scanners, some means is provided for sensing the color of each part of original item 101. In one method of sensing color multiple sets of sensor pixels are provided, each set having a filter so that the set is responsive to only a portion of the visible light spectrum. The visible spectrum includes wavelengths between about 0.4 and 0.7 microns. For example, three sets of pixels may respond to red, green, and blue light wavelengths. The digital values from the three sets are combined into a color digital image of original item 101. In another method, a single set of pixels is responsive to substantially all visible light wavelengths, but multiple exposures are made, each exposure using light made up of only a portion of the visible spectrum. For example, three exposures may be made, one each with a red illuminant, a green illuminant, and a blue illuminant. The digital values from the three exposures are combined to form a color digital image of original item 101.
FIGS. 4A and 4B show an alternative optical system often called a contact image sensor (CIS). Light 404 from lamp 401 reflects from original item 101 and is gathered by an array of gradient index rod lenses 402. Lenses 402 form a composite image of the original item on an array of sensor segments 403. Image data read by sensor segments 403 are assembled into a single digital image of original item 101. A CIS system has the advantage that it is compact, and a scanner using a CIS system can be made correspondingly small. However, a CIS system such as is shown in FIG. 4 provides very little depth of field because the image it forms is of unit magnification and because the images formed by the individual rod lenses 402 are misaligned at positions away from the object and image planes. Objects as near as 0.5 millimeters away from platen 102 may appear blurry when imaged by a CIS system. And because the image formed by lenses 402 is of unit magnification, a costly page-wide array of sensors 403 is needed.
Each conventional architecture has advantages and disadvantages that have been weighed against each other in the design of previous scanners. While these alternative architectures have been presented in the context of a flatbed scanner scanning a reflective original item 101, similar tradeoffs occur in the design of other kinds of products as well, for example in the design of multi-function products that can scan, print, copy, and perform other functions, in the design of scanners that can scan photographic slides and other transmissive original items, and in the design of “sheet feed” scanners and facsimile machines wherein the scanning optics and sensors are held stationary while an original item is transported past for scanning.